1. Field of the Invention
The present invention relates to an encoder to be used for detecting position (or movement/rotation amount) of a movable member by using a vernier method.
2. Description of the Related Art
Encoders to be used for position detection include optical encoders and magnetic encoders, both of which are constituted by a sensor to be attached to one of a movable member and a fixed (immovable) member and a scale to be attached to the other of the sensor and the scale. The scale is provided with a periodic pattern periodically transmitting or reflecting light or periodically changing intensity of magnetic field. When the movable member is moved or rotated with respect to the fixed member and thereby the sensor and the scale are relatively moved, the sensor optically or magnetically reads the periodic pattern in the scale to produce an electric signal (sensor output signal) corresponding to the periodic pattern. Calculation using this sensor output signal enables detection of position (or movement/rotation amount) of the movable member.
Japanese Patent Laid-Open No. 2011-033464 discloses an encoder including a scale that is provided with multiple (two) periodic patterns arranged parallel and having a phase difference therebetween. In this encoder, a sensor including two light receivers reads these two periodic patterns to produce two sensor output signals having a phase difference therebetween. Performing a calculation using the sensor output signals enables position detection with higher resolution.
Moreover, the inventor of the present invention researches a vernier encoder, such as one shown in FIG. 12, which uses a scale provided with two scale tracks 1008 and 1011. Each of the scale tracks 1008 and 1011 includes multiple periodic patterns arranged parallel and having mutually different periods. For example, the scale track 1008 alternately includes periodic patterns (each hereinafter referred to as “a coarse pattern”) 1111, 1121 and 1131 1112, 1122 and 1132 having long periods and periodic patterns (each hereinafter referred to as “a fine pattern”) 1112, 1122 and 11-32 1111, 1121 and 1131 having short periods. Moreover, the scale track 1011 alternately includes coarse patterns 1211, 1221 and 1231 and fine patterns 1212, 1222 and 1232. The coarse patterns forming a pair between the two scale tracks have slightly different periods from each other, and the fine patterns forming a pair between the two scale tracks also have slightly different periods from each other.
A sensor of this encoder reads the paired coarse patterns to provide two sensor output signals having long periods slightly different from each other. Then, the encoder performs a vernier operation to calculate a phase difference between the two sensor output signals and produces therefrom a vernier periodic signal having a long period different from those of the two sensor output signals. Similarly, the encoder reads the paired fine patterns to provide two sensor output signals having short periods slightly different from each other. Then, the encoder performs the vernier operation to calculate a phase difference between the two sensor output signals and produces therefrom another vernier periodic signal having a short period different from those of the two sensor output signals. The encoder uses the long period vernier periodic signal thus obtained as a high-level signal Sv1 and uses the short period vernier periodic signal as a middle-level signal Sv2, and synchronizes the high-level signal Sv1 and the middle-level signal Sv2 to enable position detection.
However, when using such a scale provided with the two scale tracks (hereinafter also referred to as “a multiple track scale”), there are the following problems. First, the movable member and the fixed member to which the sensor and the scale are attached often have mechanical backlash therebetween in a scale width direction (Y direction in FIG. 12) orthogonal to a position detection direction (X direction in FIG. 12) that is a direction in which each periodic pattern extends. The mechanical backlash causes a relative positional shift between the sensor and the scale in the scale width direction, which causes so-called cross talk in which the sensor reads the periodic pattern in the scale track different from the scale track that the sensor should read. The cross talk results in a position detection error as shown in FIG. 13.
FIG. 13 is a graph showing dependence of synchronization accuracy of the high- and middle-level signals Sv1 and Sv2 and dependence of position detection accuracy on relative positional shift amount between the sensor and the scale in the Y direction. A solid line shows the synchronization accuracy of the high- and middle-level signals Sv1 and Sv2, and a broken line shows the position detection accuracy. A horizontal axis shows the relative positional shift amount between the sensor and the scale in the Y direction.
The relative positional shift amount between the sensor and the scale in a +Y direction in FIG. 13 corresponds to a positional shift amount of the sensor with respect to the scale in an upper direction in FIG. 12, and the relative positional shift amount between the sensor and the scale in a −Y direction in FIG. 13 corresponds to a positional shift amount of the sensor with respect to the scale in a lower direction in FIG. 12. A first vertical axis on a left side in the graph shows the synchronization accuracy between the high- and middle-level signals Sv1 and Sv2. A synchronization accuracy exceeding 1 makes the synchronization between the high- and middle-level signals Sv1 and Sv2 impossible. On the other hand, a second vertical axis on a right side in the graph shows the position detection accuracy. Moreover, Y1 represents a distance between a light emitter and a light receiver of the sensor in the scale width direction (see FIG. 1B).
The cross talk caused in the high-level signal Sv1 deteriorates the synchronization accuracy of the high- and low-level signals Sv1 and Sv2 and thereby might make it impossible to synchronize the high- and low-level signals Sv1 and Sv2. In FIG. 12, when the relative positional shift amount of the sensor and scale is ±Y1/2, a sensor reading area where the sensor can read the periodic pattern includes a boundary 1031 between the two scale tracks, which causes the sensor to perform cross talk reading of the long period pattern (for example, the periodic pattern 1112) and the short periodic pattern (for example, the periodic pattern 1212) respectively included in the mutually different scale tracks.
In addition, increase of the relative positional shift amount of the sensor and scale makes the cross talk significant. Particularly, increase of the relative positional shift amount in the +Y direction increases influence of the cross talk on the high-level signal Sv1, which results in deterioration of accuracy of the high-level signal Sv1 and also results in the synchronization accuracy between the high- and middle-level signals Sv1 and Sv2. The deterioration of the synchronization accuracy causes an error corresponding to at least one period in the middle-level signal Sv2, which significantly deteriorates the position detection accuracy as in the case where the relative positional shift amount is +Y1/2.
As a countermeasure to these problems, a method can be considered which increases a distance between the light emitter and the light receiver of the sensor to enable suppression of occurrence of the cross talk even though the sensor and the scale are largely relatively shifted in the Y direction. However, this method increases in size of the sensor and thus that of the entire encoder, which is undesirable.